In general, high strength and low alloy steel represented by AISI4340,300M and the like are known well as a high strength material. These materials can provide high strength of about 180 Kgf/mm.sup.2 or more, when heat treatment conditions are selected. However, since these materials are low alloy steels and contain a small amount of Cr of 1% or less which greatly contributes to corrosion resistance, they have poor corrosion resistance. Thus, when these materials are used to an application requiring corrosion resistance, they have been conventionally subjected to a surface treatment such as Cr plating, Ni plating and the like. Nevertheless, the method of improving the corrosion resistance thereof by the surface treatment has problems in that the method needs many processes, when a surface treated layer is peeled, the peeled portion is partially corroded, and further the surface treatment is difficult to be carried out depending upon parts or locations.
On the other hand, generally stainless steel is often used to an application in which corrosion resistance is an important factor. Although stainless steel has excellent corrosion resistance, austenitic stainless steel represented by SUS304 and ferritic stainless steel represented by SUS430 and the like, which are known well, have low strength, and thus it is not suitable to an application simultaneously requiring corrosion resistance and strength. On the other hand, a precipitation hardening type stainless steel has high strength, which can be increased in such a manner that the stainless steel is age-hardened by an aging treatment. Well known commercially available precipitation hardening type stainless steel includes 17-4PH, 15-5PH, PH13-8Mo and the like. The strength of these precipitation hardening type stainless steels is about 120 Kgf/mm.sup.2 in the case of 17-4PH, about 135 Kgf/mm.sup.2 in the case of 15-5PH, and about 150 Kgf/mm.sup.2 in the case of PH13-8Mo, although it changes depending upon aging treatment conditions.
Although these precipitation hardening type stainless steels have high strength, the strength is lower than that of 4340,300M and the like as high strength low alloy steel. Therefore, these existing precipitation hardening type stainless steels cannot be used to an application in which both high strength similar to that of 4340 or 300M and corrosion resistance similar to that of stainless steel are required, from the view point of the strength, although the corrosion resistance thereof has no problem. Therefore, if there is stainless steel which has excellent corrosion resistance as well as strength similar to that of 4340 or 300M, it can be widely used to the application having strict requirements.
Further, since a high strength material has low toughness, there has been a desire to improve the toughness of the high strength material to a value as higher as possible in order to practically use the material and make use of the advantage of the high strength.
The high strength low alloy steel 4340 has the strength level of 180 Kgf/mm.sup.2 and the fracture toughness value (K.sub.IC value) of about 200 Kgf/mm.sup.2 .multidot..sqroot.mm, and further, existing precipitation hardening type stainless steels have the toughness level of about 200 Kgf/mm.sup.2 .multidot..sqroot.mm in the case of 17-4 PH and about 250 Kgf/mm.sup.2 .multidot..sqroot.mm in the case of 15-5PH and PH13-8Mo when represented by a fracture toughness value (K.sub.IC value). More specifically, the precipitation hardening type stainless steels have not always high toughness, although they have a strength level lower than that of high strength low alloy steel.
On the other hand, examples of stainless steel having high strength and relatively high toughness are disclosed in Reissue U.S. Pat. No. 26,225 as heat resistant high strength stainless steel having very high strength and in U.S. Pat. No. 3,756,808 as stainless steel, respectively. U.S. Pat. No. 3,756,808 shows in FIG. 2 that the heat resistant high strength stainless steel 77 (AFC77, C; 0.16%, Cr; 14.36%, V; 0.48%, Mo; 4.90%, Co; 13.60%, N; 0.05%, Al; 0.042%, and residue; Fe) disclosed in Re. U.S. Pat. No. 26,225 and the stainless steel AFC260 (C; 0.07%, Si; 0.25%, Mn; 0.25%, Ni; 1.85%, Cr; 15.5%, Mo; 4.5%, Co; 13.0%, Nb; 0.15%, N; 0.03%, residue; Fe) and Alloy B (C; 0.16%, Ni; 1.03%, Cr; 13.94%, V; 0.09%, Mo; 5.22%, Co; 13.67%, Nb; 0.22%, N; 0.032%, residue; Fe) disclosed in U.S. Pat. No. 3,756,808 have strength and toughness higher than those of other precipitation hardening type high strength stainless steels represented by 17-4PH, 15-5PH, PH13-8Mo and the like. It is shown that Alloy B having the highest strength and toughness levels among them has a strength of 180 Kgf/mm.sup.2 (about 260 Ksi) and the toughness thereof at this strength level is about 400 Kgf/mm.sup.2 .multidot..sqroot.mm (about 115 Ksi .sqroot.in). U.S. Pat. Nos. 3,756,808 and 3,873,378 show that the strength and toughness of this alloy greatly depend on heat treatment conditions. According to U.S. Pat. No. 3,756,808, the heat treatment conditions for providing Alloy B with such high strength and high toughness are such that Alloy B is austenitized by being kept at 927.degree. C. for 1 hour and cooled to a room temperature, then again austenitized by being heated to 1150.degree. C. and kept at this temperature for 1 hour, cooled to 1038.degree. C. in this state and kept at this temperature for 1 hour and cooled to a room temperature, and further subjected to a sub zero treatment at -73.degree. C. for 1 hour, and finally tempered twice at 427.degree. C. for 2 hours.
In the above treatments, the first austenitizing treatment carried out at 927.degree. C. is to adjust the size and distribution of a Nb carbide to thereby prevent crystalline grains from coarsening in the next second austenitizing treatment carried out at a high temperature. Further, the second austenitizing treatment carried out at 1150.degree. C. and 1038.degree. C. is to stabilize austenite at the high temperature of 1150.degree. C. and to keep delta-ferrite as a brittle phase simultaneously made at this time at 1038.degree. C. to thereby make the same disappear. When Alloy B is cooled after the austenitizing treatments, much retained austenite remains, and thus the toughness and elongation thereof can be increased. To increase the toughness and elongation by the adjustment of an amount of the residual austenite, however, the amount and distribution of the residual austenite must be well controlled, and when the size of Alloy B is large, it is feared that the control of the amount and distribution of the residual austenite may be difficult. Further, the tempering carried out at 427.degree. C. is effective to increase strength.
As the result of a specific study of alloy having the composition of Alloy B carried out by the inventors using an experimental method, as shown in an embodiment, the high strength and high toughness disclosed in U.S. Pat. No. 3,756,808 could not be obtained, even if the above heat treatment was carried out, and only a low value of proof stress was obtained. As described above, it is very difficult to simultaneously obtain both high strength and high toughness in stainless steel.